Paired glands near the base of the whip-like flagellum store the defensive fluid. The animal rotates the tip of its abdomen towards a threat and releases a jet that can travel some tens of centimetres. Analysis of Mastigoproctus giganteus spray found it dominated by acetic acid, at around 84 per cent, with octanoic acid making up most of the remainder and water the rest.
The octanoic acid matters more than its small share suggests. It is a lipid-soluble compound that breaks down the waxy layer on an insect's cuticle, letting the acetic acid penetrate rather than simply run off. Against a vertebrate the effect is irritation, particularly to eyes and mucous membranes.
Whip scorpions have no venom and no sting; the long flagellum is a sensory organ, not a weapon. The spray is the whole defence, backed by pedipalps strong enough to crush an insect. Whether the acid deters mammals as effectively as it deters arthropods has not been carefully tested.